How to fix a stiff right click button (Logitech M320)?

It could be some sort of debris that has gotten lodged under the surface where you push down to right-click, in which case you could gently lift up on the plastic surface and used compressed air to try and dislodge it.

Or it could be the actuators that are underneath the plastic casing inside the mouse itself that has gone bad.

You can remove the battery and under where the battery was inside the battery compartment is a black sticker that needs to be removed to expose a single black Philips head screw.

then carefully separate the mouse to expose the inside and examine for any debris and also check out the actuators

the two yellow arrows point to where the actuators are

note the second picture is not an actual m320 the picture is for reference only